Difference imaging, in its simplest form, is carried out by talking two separate images of the same object or scene with each image being separated by a certain duration of time (the reference and test images). When the two images or brought into spatial alignment, and the values of the individual pixels of one image is computationally compared with the other image (usually by subtraction), the result is a “difference image” where the pixel values quantify (and spatially map) the magnitude of change that has occurred in the scene during the interval between the two samples.